Are the free core nutation parameters variable in time?

The free Core Nutation (FCN) is a normal mode of the Earth which induces resonance in the diurnal tides as well as in the nutations. This is particularly true for frequencies near the resonance frequency, i.e., for the retrograde annual nutation and the psi(1) tide. The paper aims at analyzing Very Long Baseline Interferometry (VLBI) data and superconducting gravimeter (SG) data in order to gain more information on the FCN period, damping, resonance strength and free mode amplitude. Section 2 deals with the variability as seen in precise tidal gravimetric observations from consecutive subsets of an SG located near Strasbourg (France). We will show that the apparent variations appearing in the eigenperiod, quality factor and strength are related to a variable noise level in the tidal gravimetric data. Section 3 is devoted to the analysis of the time variability of the VLBI series. In particular we show that the amplitude variations found in the forced nutational response of the Earth are not real but rather induced by a variable free mode excitation. We show that the eigenperiod is stable within a range of 3 days. (C) 2000 Elsevier Science B.V. All rights reserved.